The present invention relates to a semiconductor laser and, more particularly, to a semiconductor laser characterized by a carrier block portion for an active layer.
As is well known, a semiconductor laser is a light emitting device used in a broad range of fields such as optical communication, various optical measurements, and optical recording.
The characteristics required of this semiconductor laser slightly vary from one application to another. However, high output is a common subject to all fields.
FIG. 5 shows the structure of a 1.48-xcexcm semiconductor laser for exciting an erbium-doped fiber amplifier used in the field of optical fiber communication.
In this semiconductor laser, a mesa structure is formed on an n-type InP substrate 51.
In this mesa structure, an InGaAsP separate confinement heterostructure (SCH) layer 52, an InGaAsP active layer 53, an InGaAsP SCH layer 54, and a p-type InP cladding layer 55 are stacked in this order.
The InGaAsP SCH layers 52 and 54 have a bandgap intermediate between the active layer 53 and the InP layers 51 and 55 and can have either a uniform structure or an internal refractive index distribution (to be referred to as a GRIN structure hereinafter).
The InGaAsP active layer 53 often has a multiple quantum well structure (MQW) in which quantum well layers and barrier layers are stacked, rather than a bulk structure.
In the remainder of this specification, when a term xe2x80x9cactive layerxe2x80x9d is used it doesn""t matter whether the active layer has a bulk structure or an MQW structure.
Likewise, when a term xe2x80x9cSCH layerxe2x80x9d is used in this specification, it doesn""t matter whether the SCH layer has a uniform refractive index or a GRIN structure.
A p-type InP buried layer 56 and an n-type InP buried layer 57 are buried in the two sides of the mesa to contribute to the constriction of a current path and the formation of a stripe optical waveguide.
A p-type InGaAs contact layer 58 is formed on the p-type InP cladding layer 55 in many instances.
A semiconductor layer is completed by forming metal electrodes on the two surfaces of the semiconductor crystal thus formed.
Output of a semiconductor laser like this can be increased by many factors. Among other factors, it is important to allow radiative recombination at high probability of carriers (electrons and holes) injected into the active layer.
To this end, it is desirable to block carriers overflowing from the active layer by a p-type dopant in the cladding layer and thereby enhance the confinement of carriers to the active layer.
A semiconductor laser like this often uses metal organic vapor phase epitaxy (MOVPE) having high mass productivity for crystal growth, Zn as a p-type dopant, and Si as an n-type dopant.
The doping amounts of these dopants control the carrier concentration in a semiconductor.
The higher the carrier concentration, the better the electrical characteristics. However, as particularly holes readily absorb light, caution should be exercised on the carrier concentration in the p-type cladding layer.
That is, it is necessary to take account of not only the active layer carrier blocking effect described above but also the light absorbing effect of holes.
Additionally, when Zn is used as a p-type dopant, the problem of diffusion in heating steps must be taken into consideration.
More specifically, if the concentration of a p-type dopant in a cladding layer near an active layer is high, Zn easily diffuses in the crystal. Consequently, Zn enters the active layer (a well layer in the case of an MQW structure) which is supposed to be undoped. This Zn functions as the source of non-radiative recombination and degrades the laser characteristics. This is known to those skilled in the art.
To obtain a doping concentration distribution in a p-type cladding layer by which this Zn diffusion to an active layer is prevented, Jpn. Pat. Appln. KOKAI Publication No. 7-193321 has disclosed a technique which decreases the doping concentration near an active layer and increases the doping concentration in a direction away from the active layer.
Also, to suppress unnecessary absorption of light while blocking electrons from an active layer, Jpn. Pat. Appln. KOKAI Publication No. 9-45989 uses a method in which only a thin layer near an active layer is heavily doped and layers above this thin layer are lightly doped.
This method uses C or Mg as a dopant instead of Zn which is readily diffusible.
In the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-193321, however, the dopant concentration in a cladding layer gradually increases from an active layer. This makes it difficult to sufficiently block electrons overflowing from the active layer in the vicinity of the active layer.
In this technique, therefore, electrons easily overflow from an active layer, so it is impossible to allow effective radiative recombination of a current injected into the active layer. Hence, the technique is unsuited to increasing the output of a semiconductor laser.
In the method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-45989, C or Mg must be used to suppress diffusion of a dopant to an active layer.
Unfortunately, these materials have lower purities and higher prices than those of Zn and hence cannot be generally used as a p-type dopant.
The present invention has been made in consideration of the above situation, and has as its object to provide a semiconductor laser capable of preventing diffusion of a p-type dopant to an active layer while performing sufficient carrier blocking, even when Zn is used as a p-type dopant, and obtaining high emission efficiency and high output by minimizing light absorption in a p-type cladding layer.
To achieve the above object, according to one aspect of the present invention, there is provided a semiconductor laser having a stacked structure including an n-type semiconductor substrate, an active layer, and a p-type cladding layer formed on the active layer, wherein the p-type cladding layer is formed such that regions doped with a p-type dopant are formed in an order of a first lightly doped region, a heavily doped region, and a second lightly doped region from a region closest to the active layer in a stacking direction, and a maximum value of concentration of the p-type dopant exists in the heavily doped region within a range of 50 to 250 nm in the stacking direction from the active layer.
According to another aspect of the present invention, there is provided a semiconductor laser comprising:
an n-type semiconductor substrate;
an active layer formed on the n-type semiconductor substrate; and
a p-type cladding layer formed by doping a p-type dopant into the form of a stacked structure on the active layer,
the p-type cladding layer comprising:
a p-type dopant diffusion preventing layer formed in a region near the active layer to prevent diffusion of the p-type dopant to the active layer during fabrication process of the semiconductor laser;
a carrier blocking layer formed on the p-type dopant diffusion preventing layer to block carriers overflowing from the active layer and confine the carriers in the active layer when the semiconductor laser is in operation; and
a light absorption inhibiting layer formed on the carrier blocking layer to inhibit absorption of light by holes when the semiconductor laser is in operation).
According to still another aspect of the present invention, there is provided a method of fabricating a semiconductor laser, comprising the steps of:
forming an active layer on an n-type semiconductor substrate; and
forming a p-type cladding layer by doping a p-type dopant into the form of a stacked structure on the active layer,
the step of forming the p-type cladding layer comprising:
forming a p-type dopant diffusion preventing layer in a region near the active layer to prevent diffusion of the p-type dopant to the active layer during a fabrication process of the semiconductor laser;
forming a carrier blocking layer on the p-type dopant diffusion preventing layer to block carriers overflowing from the active layer and confine the carriers in the active layer when the semiconductor laser is in operation; and
forming a light absorption inhibiting layer on the carrier blocking layer to inhibit absorption of light by holes when the semiconductor laser is in operation.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.